Ink jet printing is a non-impact method for producing printed images by the deposition of ink droplets in a pixel-by-pixel manner to an image-recording element in response to digital data signals. There are various methods that may be utilized to control the deposition of ink droplets on the image-recording element to yield the desired printed image. In one process, known as drop-on-demand ink jet, individual ink droplets are projected as needed onto the image-recording element to form the desired printed image. Common methods of controlling the projection of ink droplets in drop-on-demand printing include piezoelectric transducers and thermal bubble formation. In another process, known as continuous ink jet, a continuous stream of droplets is charged and deflected in an image-wise manner onto the surface of the image-recording element, while un-imaged droplets are caught and returned to an ink sump. Ink jet printers have found broad applications across markets ranging from desktop document and photographic-quality imaging, to short run printing and industrial labeling.
The inks used in the various inkjet printers can be classified as either dye-based or pigment-based. A dye is a colorant that is dissolved in the carrier medium. A pigment is a colorant that is insoluble in the carrier medium, but is dispersed or suspended in the form of small particles. These small particles can be stabilized against flocculation and settling by the use of distinct dispersing agents such as surfactants, oligimers or polymers, or they can be directly functionalized to provide a self-dispersing characteristic. In either case the carrier medium can be a liquid or a solid at room temperature. Commonly used carrier media include water, mixtures of water and organic co-solvents and high boiling organic solvents, such as hydrocarbons, esters, ketones, alcohols and ethers.
Pigment-based inkjet inks are often preferred over dye-based inkjet inks because of the superior image stability typically observed with the pigment-based inks. Self-dispersed pigments in turn are often preferred over surfactant-dispersed, oligimer-dispersed or polymer-dispersed pigments because of their greater stability to a variety of ink formulations and environmental keeping conditions.
Several workers have reported the preparation of covalently functionalized self-dispersed pigments suitable for ink jet printing. Bergemann, et al., in U.S. Pat. No. 6,758,891 B2 describe the covalent functionalization of pigments by reaction with organic triazoles. Bergemann, et al., in U.S. Pat. No. 6,660,075 B2 further describe the covalent functionalization of pigments by reaction with unsaturated organic compounds. Belmont in U.S. Pat. No. 5,554,739, Adams and Belmont in U.S. Pat. No. 5,707,432, Johnson and Belmont in U.S. Pat. Nos. 5,803,959 and 5,922,118 and in published applications WO 96/18695, WO 96/18696, WO 96/18689, WO 99/51690, WO 00/05313, and WO 01/51566 describe the covalent functionalization of pigments with diazonium compounds. Like preparations of covalently functionalized self-dispersed pigments are additionally described by Osumi et al., in U.S. Pat. Nos. 6,280,513 B1 and 6,506,239 B1. These publications further describe the preparation and use of ink-jet inks employing the described self-dispersed pigments. Both anionic and cationic self-dispersed pigments are described. Karl, et al., in U.S. Pat. No. 6,503,311 B1 and Yeh, et al., in U.S. Pat. No. 6,852,156 B2, have described anionic self-dispersed pigments prepared by ozone oxidation. Ito et al., in U.S. Pat. No. 6,488,753 B1 and Momose et al., in EP 1,479,732 A1, describe anionic self-dispersed pigments prepared by hypochlorite oxidation.
While these self-dispersed pigments provide many advantages as colorants over other known colorants, they still suffer from inter-color mixing, the so called inter-color bleed problem, when applied in close proximity one to another on a media to form a multicolor image or graphic. This problem can be particularly severe when the media chosen is an inexpensive, consumer target, plain paper.
Many solutions have been proposed to the inter color bleed problem in inkjet ink printing and imaging. Rehman, in U.S. Published Application 2003/0079651, describes the incorporation of salts in some dye-based inks of a multicolor ink set to destabilize colorants in another dye-based ink of the same set when the inks are applied in close proximity on an imaging medium. Specific inter-color bleed problems can be reduced in this manner. Interestingly, Osumi et al., U.S. Pat. No. 6,280,513 B1, describe inkjet inks employing self-dispersed carbons directly incorporating effective levels of salts to provide improved density and fiber coverage on plain papers. However, direct incorporation of the salts in the pigment-based ink does not inherently provide bleed control and further tends to cause self-dispersed pigment-based inks to be unstable to long term keeping. Rehman, in U.S. Pat. No. 6,187,086 B1, describes the use of specific ink solvents in pigment-based inks of a multicolor ink set to destabilize colorants when dye-based inks of the set are applied in close proximity to an imaging medium. Lin, U.S. Pat. No. 6,475,271 B2, describes the incorporation of low boiling organic solvents in particular inks of an inkjet ink set to promote rapid drying and thus reduce inter-color bleed. Yamazaki, et al., U.S. Patent 2004/0,123,774 A1, describes the choice of high carbon content solvents in particular inks of an inkjet ink set to promote ink localization on chosen media and thus reduce inter-color bleed. Lin, U.S. Pat. No. 6,383,274 and Ma, et al., U.S. Pat. No. 6,436,180 B1, describe the use of perfluoroalkyl surfactants to selectively adjust the surface tension of specific inks in an inkjet ink set for the purpose of reducing inter-color bleed. Hayashi, U.S. Pat. No. 6,500,248 B1, describes the incorporation of effective quantities of dynamic surface tension (DST) reducing surfactants in text black inks for the purpose of reducing inter-color bleed. Unfortunately, addition of such DST control agents to self-dispersed carbon containing inks further containing salts only exacerbates the already present ink stability problem.
Thus, while several approaches to controlling the inter-color bleed problem have been reported, the approaches tend to cause inherent ink instability issues on storage and keeping. Further, the formation of high, uniform and consistent single color densities on a variety of plain papers as well as designed ink-jet papers while simultaneously controlling inter color bleed not been adequately addressed.